Dopamine is a substance produced naturally by neurons in the basal ganglia of the brain that allows smooth, co-ordinated control of voluntary movement. Loss of, or impairment of, dopamine-producing neurons in the brain is implicated in Parkinson's disease and related parkinson-plus syndromes. These conditions respond to dopamine replacement therapy. Other conditions, for example, Restless Legs Syndrome (RLS) also respond to dopamine replacement therapy.
RLS is a neurosensorimotor disorder with parestethesias, sleep disturbances and, in most cases, periodic limb movements of sleep (PLMS). Two forms of RLS appear to exist: the idiopathic and the uremic form. RLS is characterised by (1) a desire to move the legs, usually associated with paresthesias/dysesthesias, (2) motor restlessness, (3) worsening or exclusive presence of symptoms at rest (i.e. lying, sitting) with at least partial or temporary relief by activity, and (4) worsening of symptoms during the evening or night.
Parkinson's disease is a progressive neurodegenerative disorder that affects neuronal cells in the substantia nigra in the mid-brain. It is an age-related disorder of the central nervous system primarily attacking people over the age of 60. Approximately one out of every 500 people contract the illness and approximately one out of every 100 people over the age of 60 develop the illness. As indicated above, Parkinson's Disease is thought to be caused by a deficiency of dopamine. The common symptoms include tremor, stiffness (or rigidity) of muscles, slowness of movement (bradykinesia) and loss of balance (postural dysfunction). Parkinson's Disease is one of the most prevalent neurodegenerative illnesses. The natural history of the disease is progressive and from 10-15 years from onset of the disease becomes disabling in most patients.
Parkinson's disease is largely sporadic and referred to as idiopathic in nature. Forms of the illness due to vascular incidents and to toxin exposure also exist. Rare familial forms of the illness also exist.
Many treatments have been tried since James Parkinson first described the condition in 1817. Current therapy for Parkinson's disease is based on dopamine replacement therapy based on the use of the dopamine precursor levodopa (or L-dopa) or dopaminergic compounds. L-dopa is highly effective in reversing the motor symptoms of the illness but on chronic treatment and with disease progression, its effectiveness declines. The duration of drug response is reduced and unpredictable fluctuations in movement occur. Treatment is associated with therapy limiting side effects which include involuntary movements (dyskinesia) and psychosis.
There is therefore a need for L-dopa prodrugs for use in dopamine deficiency diseases, which are converted in vivo to L-dopa and result in prolonged, effective exposure to L-dopa, thereby prolonging the response to the drug.